Email received November 25 2008 from Anthony Wilson of the Arbovirus Modelling and Entomology Epidemiology Division, Institute for Animal Health
First of all, thank you for putting so much time and effort into the
website; it is a very valuable resource and I read it regularly with
great interest. Last week I spoke to the Goat Veterinary Society at
Chepstow on the epidemiology of bluetongue, and I would like to clarify
a couple of items from my presentation which have since been reported
via your site.
Regarding the origin of the BTV-6 strain which was recently isolated in
northern Europe, the sequencing work done so far by the Arbovirus
Research Group at Pirbright (headed by Peter Mertens) strongly suggests
that at least part of the virus is derived from the South African
vaccine strain; this strain is used as part of the multivalent SA
vaccine and as part of a different multivalent vaccine in Israel.
However, the absence of antibodies to any other serotypes in the tested
animals indicates that they had not themselves been vaccinated with
either of these - this was not a "first generation" escapee. A more
detailed description of this work can be found on ProMED (report
There are therefore two possibilities - the virus escaped outside Europe
and was then imported, or the virus escaped following vaccine use within
Europe. While we cannot absolutely rule either of these scenarios out,
BTV-6 does not occur in the same bottle of the SA vaccine as BTV-8,
while BTV-8 does not occur at all in the Israeli vaccine; there would
therefore be no reason why anyone would want to use a vaccine containing
BTV-6 in the BTV-8 zone. Furthermore, as we saw in 2006, a route for the
introduction of exotic BTV strains into northern Europe does exist (even
if we have not yet identified it), so taking all the above into account
the former seems more plausible to me. This is however purely
speculation on my part.
Further sequencing work is currently being undertaken by Peter's group,
and when these results are available we should have a better idea of how
long it has been since the virus "escaped" from the vaccine and
potentially how it got into Europe.
Secondly, we still do not know how BTV-8 got into northern Europe -
there is no "smoking gun", but there are a number of possibilities (each
relatively unlikely, unfortunately). The "flower theory" arose from the
observation that many serotypes of BTV circulate in sub-Saharan Africa,
and that cut flowers from these regions may be packed at night, under
brightly-lit, open-sided buildings; these buildings are therefore
qualitatively similar to the light traps we use to trap midges, but much
larger. The packed containers are wrapped in polythene and chilled to
keep the flowers fresh, and midges are capable of surviving for
relatively long periods under cool conditions. This route has therefore
yet to be ruled out, but is far from being confirmed.
Incidentally, this theory should not be confused with the idea that
midges can be introduced in the water of "lucky bamboo" and similar
plants; while Aedes mosquitoes can be introduced by this route,
Culicoides do not breed in this type of habitat, and BTV does not appear
to spread vertically in the insect vectors - this route can therefore be
conclusively ruled out.
I would also like to take the opportunity to thank Nick and the rest of
the GVS for such a well-organised and interesting meeting.
Arbovirus Modelling and Entomology
Institute for Animal Health
Ash Road, Pirbright
Woking GU24 0NF